The present invention relates to an electrical cable that is terminated to an electrical component and a method for preparing such cable for termination. More specifically, the invention is related to an electrical cable and method for termination for high-speed signal transmission.
Shielded electrical wire cable and coaxial cable are commonly used for high-speed signal transmission, and more specifically, high-speed digital differential transmission. However, the method and structure by which these cables are terminated to the components can vary. In one conventional termination structure, disclosed in Japanese Patent Application Kokai No. H10-223051, the intermediate portions of two sheet-form shielded electrical wires are bundled into a single unit by means of a tape or heat-shrink tube. Both end portions of the wires, exposed for the purpose of branching, are shielded by shielding layers of the respective electrical wires themselves. In another conventional structure, disclosed in Japanese Patent Application Kokoku No. S56-24447, a coaxial cable wrapped with a cordel braided insulator is stripped when a metal contact plug fastening fitting is attached to the terminal part of the coaxial cable. The exposed inner conductor is covered with a tightly adhering heat-shrinkable plastic tube to form a dielectric layer. The length and thickness of the dielectric layer can be arbitrarily set in order to adjust the characteristic impedance of the internal conductor.
In the first recited reference the end portions of the shielded electrical wires are merely shielded in the shielded electrical wire terminal treatment structure and, therefore, impedance matching cannot be maintained. Further, because impedance matching is accomplished by means of a heat-shrink tube in the coaxial cable method, recited in the second reference differential transmission signal wires are not accounted for. Thus, in these conventional methods, the individual differential transmission signal wires and drain wire from which the shielding covering has been stripped are generally connected to a corresponding circuit board, etc., in a loose state, i.e., in a state in which the positional relationship of the electrical wires is not maintained. The end portion of the shielding covering and the portion of the drain wire that is exposed from the end portion are separately covered by tightly adhering heat-shrink tubes. For that reason, the characteristic impedance of the exposed differential transmission signal wires is not matched causing problems such as signal reflection and signal deviation. This terminal treatment is expensive and time consuming.
It is therefore desirable to develop an inexpensive electrical cable structure and method where the matching (balance) of the differential impedance between the two differential transmission signal wires and the drain wire is maintained so that there is little signal reflection when the cable is terminated to an electrical component. It is also desirable to develop a structure and method that is suitable for use in high speed transmission and prevents deterioration of the transmission characteristics by eliminating skewing (propagation delay time difference, i.e., signal deviation) between the two signal wires.
The invention relates to an electrical cable terminal part structure having two differential transmission signal wires. An insulator covers the respective core wires. A single drain wire is disposed adjacent to the differential transmission signal wires at an equal distance from each of the differential transmission signal wires. A shielding covering surrounds the wires. The area around the two differential transmission signal wires and the drain wire are exposed at a terminal part or front end of the electrical cable by stripping the shielding covering. Except for the exposed front end portions of the differential transmission signal wires and drain wire, the area around the two differential transmission signal wires and the drain wire is covered by a heat-shrink tube together with the end portion of the shielding covering, so that mutual distances between the wires inside the cable are maintained.
This invention also relates to a method in which the two differential transmission signal wires and the drain wire are exposed by stripping the shielding covering at the terminal part or termination end of the electrical cable. The area around the two differential transmission signal wires and the drain wire that are exposed is covered with a heat-shrink tube together with the end portion of the shielding covering. Only the front-end portions of the differential transmission signal wires and drain wire remain exposed so that mutual distances between the wires inside the electrical cable are maintained.